Did Comey’s October announcement really change the course of the election? In fact we don’t know. However you found it useful to overstate the case and took Nate Silver’s. “… might have shifted…” and strengthened it to arrive at your “almost certainly changed the course of American history”.

Even Nate Silver’s case is conjectural and suffers from the post hoc logical fallacy. Just because his polling data changed after the letter doesn’t mean the letter caused the change. By similar logic it was the Cubs getting into the World Series, the huge Obamacare market rate increases that came out in October, the falling of autumn leaves, or anything else that came before.

Until someone releases exit polls where voters say: 1. yes I switched my vote to Trump and 2. The Comey letter was the key to making me switch, the whole argument is rank speculation.

I choose to believe the release of the Comey letter helped Clinton. It didn’t persuade anyone to change their mind, but it did persuade marginal Clinton supporters they needed to vote and thereby raised her turnout. Without the Comey letter she would have lost by another 3–4 points. What, you don’t believe me? Fine, but you have no data to prove me wrong.

Most persuasive is that at the time Hillary and her team didn’t think it would have much effect. Otherwise she would have scheduled a few last minute campaign stops and shifted ad money to the Mid-west.